Optical sensor technology can be used to detect a large range of chemicals including CO2, CO, NO2, VOCs and alcohol. For example Nondispersive infrared (NDIR) type sensor systems can offer good sensitivity, stability and selectivity. They have many applications, including ambient air quality and safety monitoring. Similarly Attenuated Total Reflection (ATR) systems also use IR radiation to determine the composition of a chemical.
Infrared gas sensors exploit the principle of optical absorption. When infrared radiation passes through a gas, some of the optical energy is absorbed and transitions occur between the vibrational-rotational energy levels within the gas molecules. This process creates ‘absorption lines’ in the mid infrared spectrum (generally between 2.5-16 μm). The characteristics of the absorption spectra depend on the number and masses of atoms in the molecules, as well as the nature of the various chemical bonds. A commonly detected gas is carbon dioxide (CO2) which has strong absorption lines at a wavelength of 4.26 μm.
A basic single channel NDIR system consists of a broadband infrared source which emits infrared radiation through a gas cell. An optical bandpass filter is used to select the absorption wavelength of interest and an infrared detector detects the transmitted IR signal. When the target gas concentration is low, there is limited interaction between the optical signal and the gas molecules and the detected signal is therefore high. If the target gas is introduced, optical absorption occurs and the detected signal level drops in proportion to the gas concentration. The transmitted optical intensity is described by the Lambert and Beer lawI=I0e−kcl where I0 is the initial intensity, k is the gas specific absorption coefficient, c is the gas concentration and l is the length of the optical absorption path.
The infrared source used for the system can be a broadband thermal emitter such as an MEMS infrared source, infrared incandescent lamp or blackbody radiation source. Alternatively, a narrow band source, such as an infrared diode or laser can be used. The choice depends on a number of factors, including the optical power, spectral characteristics, cost and frequency response. Typically, micro-bulbs are used which have the advantage of being extremely cheap and provide good emission at short mid-IR wavelengths. Disadvantages are that they have high power consumption, are bulky, and have limited emission at longer wavelengths (>5 μm) due to optical absorption by the glass envelope. As a result, MEMS IR emitters are increasingly being used which consist of a micro-heater embedded within a membrane that is thermally isolated from a silicon substrate. These can offer good performance across a broader range of wavelengths and better integration with other chip based technology.
Two types of IR detector are available: thermal and quantum detectors. Thermal detectors respond to the heating of a material and include: bolometers, thermopile and pyroelectric detectors. They typically have a broadband response in the mid IR waveband and have to be used with optical filters for gas sensing applications. Quantum type detectors, such as photodiodes and photoconductive sensors, are fabricated from semiconductor materials which determine their spectral response. For most optical gas sensing applications, thermal detectors are used as they give adequate performance, do not require cooling, are lower cost, and can be integrated on a semiconductor chip.
In a refinement of the basic NDIR approach described, a dual channel system can be used to help compensate for the effect of system drift, for example, due to variations in IR emission from the source over time. With this approach, a second ‘reference’ detector is used with a detection wavelength well away from the absorption wavelength of the target gas. Often dual channel detectors are integrated within the metal package of a single component.
A number of NDIR sensor designs are known, for example U.S. RE36277, U.S. Pat. No. 8,471,208, US 20080035848, U.S. Pat. No. 7,449,694, U.S. Pat. No. 6,469,303, U.S. Pat. No. 6,753,967 and U.S. Pat. No. 7,609,375. However, they all require an optical assembly which cannot be easily integrated with existing semiconductor chip technology. This precludes their use in many applications where a small form factor is required, such as for mobile phones.
In U.S. Pat. No. 5,834,777, Wong discloses a miniature NDIR gas sensor made in semiconductor technology which results in a small size, even including the optical path. In this design there is a substrate with both an IR emitter and IR detector, and a second substrate which is etched to form an optical path, and the two substrates joined together to form an optical waveguide. A diffusion type gas sample chamber is formed within the waveguide and interposed in the optical path between the light source and the light detector. However the device has a large bulk-etched substrate portion below and above the optical path. Because of its closeness to the IR emitters and detectors, it is not possible to have these on a dielectric membrane. As a result the emitter has to be either a photodiode—which has low emission and stability issues, or can be a heater but with a high power consumption. Similarly the IR detector cannot be on a membrane, and so has much lower sensitivity.